A multilayer coating totaling four different layers (four-stage system) is generally used for the production painting of automobile bodies; these four layers are applied in succession at separate paint stations:
The first layer located directly on the vehicle sheet metal is an electrophoretically applied layer (electrocoat, cathodic dip coating layer) which is applied by electro dip coating—mainly cathodic dip coating—for corrosion protection and subsequently baked.
The second layer, on top of the electro coat, is a primer-surfacer about 30 to 40 μm thick which on the one hand offers protection against mechanical attack (stone chip protection) and on the other hand ensures adequate topcoat holdout, i.e. it smoothes the rough surface of the body for the following topcoat and fills minor irregularities. The paints used to produce this primer-surfacer coat contain pigments as well as binders. The wettability of the pigments used has an effect on the topcoat holdout of the entire multilayer coating and on the gloss of the primer-surfacer, as demanded by some automobile manufacturers. Application of the primer-surfacer coat is generally carried out through application with electrostatic high-speed rotary bells and subsequent baking at temperatures above 130° C.
The third layer, on top of the primer-surfacer, is the basecoat, which gives the desired color to the body through the corresponding pigments. The basecoat is applied in a conventional spraying method. The film thickness of this traditional basecoat is between about 12 and 25 μm, depending on the hue. This coat, particularly with metallic effect paints, is mostly applied in two steps. In a first step the paint is applied by means of electrostatic high-speed rotary bells, followed by a second application by means of pneumatic atomization. This coat (when using water-based basecoat) is interim dried using infra-red heaters and/or warm air convection.
The fourth and topmost layer, on top of the basecoat, is the clearcoat, which is usually deposited in one application by electrostatic high-speed rotary bells. It gives the body the desired gloss and protects the basecoat from the effects of the environment (UV radiation, salt water, etc.).
Finally, the basecoat and the clearcoat are baked together.
Additional basic requirements besides its color-imparting properties are placed on a waterborne basecoat which can be used in this multilayer coating, or in a basecoat produced from it:
For one, the basecoat in its cured state must result in an optimal orientation of the aluminum flakes used as effect pigments. This property, known by the term “flop effect”, is of crucial importance for any metallic finish. A particularly good “flop effect” is achieved when the tiny platelet-shaped effect pigments are aligned as evenly as possible at a shallow angle to the paint layer.
In addition, the basecoat layer must have a precisely specified adhesion to the paint layers below and above it. The basecoat decisively affects the stone chip resistance of the originating multilayer coating of production automobile bodies. It should be noted in this connection that stone chip resistance is known as a “k.o. criterion,” i.e. only those multilayer coatings which have previously passed the VDA stone chip test can be used in production operations. The final multilayer coating passes the test if, under a precisely defined mechanical load, it exhibits pitting which does not exceed a certain area and is attributable to a separation of the basecoat from the primer-surfacer coat underneath it. Consequently, the adhesion of the basecoat must be adjusted in such a way that it is high enough so that the clearcoat does not separate from it, but is low enough not to pull the primer-surfacer with it when chipped by a stone, which would otherwise result in considerable corrosion damage to the automobile body.
Secondly, the basecoat must have good workability. This means that, if possible, a high enough film build can be achieved in one pass so that adequate hiding is ensured. If only 17 μm thickness is required in the basecoat for black, a color which hides well, it is at least 45 μm for white, a color which does not hide well. Applying a film thickness like this in one pass is still a considerable problem since the waterborne basecoat must possess the appropriate rheological properties.
In the case of basecoats with metallic effect pigments, the previously described problem, i.e. ensuring adequate stability with a typical film build of about 18 μm, is particularly prominent. Silver metallic is a particularly critical color in this respect.
The term “rheological properties” is understood to signify that, on the one hand, the paint has such a low viscosity in the spraying process, i.e. at high shear rates, that it can be atomized easily and, on the other hand, when it strikes the substrate, i.e. at low shear rates, it has such a high viscosity that it is sufficiently stable and does not create sags. The higher the layer thickness is to be, the greater the problem of combining these contradictory properties. The creation of a definite metallic effect is associated with these properties.
This basic problem is probably also the reason why a large number of publications are concerned with specially formulated binder systems or with special additives for waterborne basecoats.
Special additives are described (EP-0 281 936) to improve rheological properties and to create a better metallic effect. These are special coating silicates which contain substantial quantities of alkali or alkaline earth ions. These ions often lead to poor condensation water resistance in the total system of an automobile coating because of their hygroscopic effect.
So the paint manufacturers take pains to avoid such additives if possible and to use those polymers as binders which naturally incorporate the desired properties, the so-called “tailor-made” polymers.
Among the most important representatives of this type are crosslinked polymer microparticles present in an aqueous dispersion, called “microgels” for short.
The addition of microgels not only brings about an improvement in rheological properties but also has a considerable effect on the stability of the paint to be applied, the alignment of the effect pigments and the adhesion of the basecoat to the primer-surfacer below it. Thus the addition of microgels has a decisive effect on the stop chip resistance of the multilayer coating. However, it must be pointed out that not all the aforementioned properties are influenced positively by the addition of microgels:
Special microgels are known from EP 0 030 439 B1 and EP 0 242 235 A1. The aqueous microgel dispersions described there as beneficial for metallic finishes as well are not completely crosslinked microgels but belong to the so-called “core/shell” microgels.
The term “core/shell structure” is understood to signify that the polymer particle is built up essentially of two different zones: the inner zone (core) is surrounded by an outer zone (shell), where these zones have a different chemical composition and as a result different physical properties as well.
The core of this microgel can be obtained from a mixture which contains difunctional monomers in addition to monofunctional monomers. The crosslinking takes place through the use of an emulgator. This crosslinked microparticle in accordance with EP 0 030 439 B1 is subsequently coated with a layer of non-crosslinked acrylic polymers and grafted. According to EP 0 242 235 A1, the crosslinked microparticle is coated with a layer of polymerizable aromatic compounds.
It is further described in EP 0 030 439 A1 to react the microgels present in an aqueous dispersion into a non-aqueous phase and to use them for solvent-containing coating compositions.
From EP 0 038 127 B1, EP 0 029 637 A1 and GB 2 159 161 A microgels are known which are obtained through polymerization of suitable monomers in the presence of an emulgator, for example N,N-bis(hydroxyethyl)taurine.
The term “emulgator” is understood to signify those compounds which have both a hydrophilic and a hydrophobic residue. Emulgators bring about a stabilization of emulsions, i.e. of dispersed systems of two non-miscible or only partially miscible fluids or phases, one of which is finely dispersed in the other. A broader definition of such compounds is given, for example in Römpp's Encylopedia of Chemistry (vol. 2, 8th edition, 1981, pp. 1126-1127). Generally a distinction is made between ionic, non-ionic and amphoteric emulgators. For color-imparting coating compositions, emulgators are used which have a group originating from sulfonic acid as the hydrophilic residue and a longer-chain fatty alkyl residue as the hydrophobic residue.
A serious drawback to the microgels produced with the use of an emulgator is that the emulgator remains in the finished microgel; the latter can be used only with considerable disadvantages for a large number of applications, for example because of the sulfur-containing groups (sulphonic acid groups) present in the emulgator. Because of the emulgator these microgels contain, they have disadvantageous properties, for example with respect to their use in waterborne basecoats in the automobile industry, specifically with regard to storage in water and condensation water resistance.
EP-0 502 934 also describes a microgel dispersion. It is used both to improve rheological properties and to increase the gassing stability of aqueous metallic basecoats. These microgel dispersions are produced through single-stage aqueous phase polycondensation of a polyester polyol with an amino plastic resin (melamine resin).
The use of this microgel in basecoats in the painting of automobile bodies has the disadvantage that the adhesion between the basecoat and a clearcoat applied over it consisting of a powder clear coat or a powder clearcoat slurry does not meet the requirements specified by the automobile industry.
Microgels are further known from DE 195 04 015 A1 which are produced by polymerizing an ethylenically monofunctional compound (polyacrylate) with at least one ethylenically di- or multi-functional compound in the presence of a polyester. The polyester functions as emulgator and stabilizer.
These microgels have the disadvantage that the rheological properties of these paints no longer meet the increased requirements of the automobile industry. This is shown particularly clearly with respect to the requirements for viscosity on the one hand and stability on the other.
So it is not possible, using these microgels, to prepare an aqueous basecoat which has a maximum viscosity of 120 mPa·s at a shear rate of 1,000 s−1 and is so stable that the necessary coating thicknesses of 20-30 μm (depending on the particular color also less or more) can be attained without sagging.
Furthermore, microgels are known in WO 00/63265 and WO 00/63266 which can be obtained from a multi-stage polymerization process, in which polymerization of ethylenically monofunctional compounds with ethylenically di- or multi-functional compounds is carried out in a first step in the presence of a polyester polyol, polyurethane and/or polyacrylate. As a final step, the product obtained in this way is reacted with a cross-linker. However, there has been shown to be a risk of gelling in the reaction of trimellithic acid or its anhydride with the poly(meth)acrylate.
A further problem in the use of these subsequently crosslinked microgels is that waterborne basecoats containing these microgels do not demonstrate sufficient adhesion on plastic substrates to be applied directly to a plastic surface, an automobile bumper for example, without an intermediate or adhesion primer coat.